(1) Field of the Invention
The present invention relates to a wave receiving device for use in a system for measuring a distance up to an object using a wave signal represented by a sound signal including an ultrasonic signal, and a method of determining the reception of a wave signal, which is to be performed in such a wave receiving device.
(2) Description of the Related Art
There have been presented various presentation systems utilizing projectors. One of the known presentation systems is an electronic presentation system for enabling the user to write information such as characters on an image projected by the projector, using a pointing device such as an electronic pen or the like. The electronic presentation system detects the position of the pointing device on the projected image according to an ultrasonic distance measurement process.
JP-A 2001-125741 discloses a presentation system wherein a pointing rod as a pointing device has on its tip end an ultrasonic transmitter for transmitting an ultrasonic signal, and ultrasonic receivers disposed in at least three reference positions on a screen receive the ultrasonic signal transmitted by the ultrasonic transmitter. The position of the tip end of the pointing rod on the screen is detected based on the difference between the arrival times of the ultrasonic signal measured by the respective ultrasonic receivers, i.e., the times consumed until the ultrasonic signal transmitted by the ultrasonic transmitter reaches the ultrasonic receivers.
There is also known a system wherein a plurality of receivers for receiving an ultrasonic signal from an electronic pen are provided on a projector. The three-dimensional position of the electronic pen on a screen is measured based on the difference between the arrival times of the ultrasonic signal that are measured by the respective receivers.
In electronic presentation systems as described above, a conventional ultrasonic receiving device determines that it has received the ultrasonic signal simply when the amplitude of the ultrasonic signal exceeds a threshold value. Therefore, the ultrasonic receiving device suffers the following problems:
The ultrasonic receiving device has its received signal level variable depending on the distance from itself to the ultrasonic transmitting device. If the ultrasonic transmitting device has an ultrasonic transmitter on its tip end, like an electronic pen, then the ultrasonic transmitting device has an ultrasonic signal radiating pattern which is lower in level at a rear end thereof. In this case, the received signal level in the ultrasonic receiver varies depending on the orientation of the ultrasonic transmitting device with respect to the ultrasonic receiver.
FIG. 1 of the accompanying drawings schematically shows the determination of ultrasonic signal reception timing in a conventional ultrasonic receiving device at the time the received signal level varies. FIG. 1 illustrates the waveform of a received ultrasonic signal when its amplitude is normal at (a) and also illustrates the waveform of a received ultrasonic signal when its amplitude is doubled at (b). When the amplitude of the received ultrasonic signal is normal as shown at (a) in FIG. 1, since the level of noise N (disturbance noise) is smaller than a threshold value T, the ultrasonic signal reception timing can be determined by comparing the amplitude of a first wave with the threshold value T. However, when amplitude of the received ultrasonic signal is not normal as shown at (b) in FIG. 1, since the level of noise N is substantially the same as the threshold value T, the ultrasonic signal reception timing may possibly be determined in error due to the noise N.
The ultrasonic receiver is basically similar in structure to a microphone. Consequently, the ultrasonic receiver tends to detect sporadic noise or a wide range of noise, e.g., voice, the sound of a closing door, or noise generated when a display monitor is turned on, etc. If any of these noises is contained in the received ultrasonic signal, then the ultrasonic signal reception timing may possibly be determined in error.
As described above, the conventional ultrasonic receiving devices are problematic in that they may possibly determine the ultrasonic signal reception timing in error and may possibly operate erroneously due to noise.